This invention relates to a fuel cell and more particularly to the electrolyte matrix of the fuel cell and a method for manufacturing the same.
A fuel cell is a device intended to generate direct current by causing an easily oxidizable gas (fuel gas) such as hydrogen and an oxidizing gas such as oxygen to electrochemically react witn each other in a proper electrolyte. That type of fuel cell which is put to practical application is constructed by stacking a large number of unit cells with an interconnector set therebetween. Each unit cell comprises a pair of gas diffusion electrodes and an electrolyte matrix which holds an electrolyte such as a phosphoric acid and is interposed between said paired gas diffusion electrodes. A fuel gas supplied to the outer surface of the gas diffusion electrode and an oxidizing gas brought to the outer surface of the other gas diffusion electrode are made to react in the each electrolyte-electrode interface, thereby generating a direct current. The inside of the paired gas diffusion electrodes are generally loaded with a catalyst such as platinum in order to accelerate the above-mentioned reaction.
The performance property of the fuel cell of the above-mentioned type is often governed by the quality of the electrolyte matrix used. For the stable operation of the fuel cell, therefore, the electrolyte matrix is required to meet the undermentioned requirements.
(i) The electrolyte matrix should be stable chemically and thermally under an operating condition.
(ii) The electrolyte matrix should contain a sufficiently large amount of electrolyte, and further retain a great capacity to hold the electrolyte.
(iii) The electrolyte matrix should have a high hydrogen ion conductivity.
(v) The electrolyte matrix should act as insulator of electrons.
(v) The electrolyte matrix should have a sufficiently high bubble pressure to suppress mutual diffusion between the fuel gas and oxidizing gas.
The conventional fuel cell generally comprises an electrolyte matrix formed of a single layer. However, such single layer type electrolyte matrix fails to fully meet the above-listed requirements, thus decreasing the reliability of the conventional fuel cell and also output voltage thereof.
The conventional single layer type electrolyte matrix is constructed by coating phosphoric acid-resistive fine powder of, for example, silicon carbide or zirconium oxide on the catalyst layer mounted on the gas diffusion electrode. In particular, the conventional process of manufacturing an electrolyte matrix comprises the steps of:
mixing the proper amounts of silicon carbide, binder of fluorocarbon polymer such as polytetrafluoroethylene, water and other solvents;
applying the mixture over the surface of the catalyst layer coated on the gas diffusion substrate by means of, for example, rolling, spraying or screen printing;
drying the mixture to remove the solvent, thereby producing a matrix body; and finally
impregnating the matrix body with electrolyte such as phosphoric acid.
However, the above-mentioned electrolyte matrix-manufacturing method has the drawbacks that though the application of a smaller amount of binder facilitates the impregnation of the electrolyte in the matrix body, cracks tend to appear in the matrix body when heat is applied to remove the solvent or the gas diffusion electrode is handled; and the occurrence of cracks in the matrix body results in a decline in the bubble pressure of the electrolyte, gas utilization rate and the performance of a fuel cell. Further, difficulties accompanying the conventional electrolyte matrix-manufacturing method are that though application of a larger amount of the binder can suppress the appearance of cracks in the matrix body, the hydrophobicity of the binder prevents the electrolyte from being fully carried into the matrix body, thereby decreasing the conductivity of hydrogen ions.
In view of the difficulties experienced in the conventional electrolyte matrix-manufacturing method, the present inventors proposed the method of manufacturing the electrolyte matrix which comprised the steps of mixing silicon carbide, binder and phosphoric acid in the form of paste and spreading the paste over the surface of the gas diffusion electrode. However, the above-mentioned electrolyte matrix-manufacturing method previously proposed by the present inventors which indeed proved prominently useful is still accompanied with the drawbacks that a larger content of phosphoric acid in the paste intended for improvement on the hydrogen ion conductivity of the electrolyte matrix leads to a rise in the fluidity of the paste. This increased paste fluidity is accompanied with further problems that when a unit cell is constructed by compressing a pair of gas diffusion electrodes with the paste interposed therebetween or after this step, the paste leaks crosswise from the unit cell. Such objectionable event results in the difficulties that the electrolyte matrix is reduced in thickness; the bubble pressure of the electrolyte decreases; and partial short-circuiting takes place between the paired gas diffusion electrodes. After all, the previous method of manufacturing an electrolyte matrix from the above-mentioned paste was still accompanied with the drawback that the fuel cell eventually decreased in performance.